Bolt employing shearing stress for self-locking

ABSTRACT

A locking threaded bolt is provided whose head has an inclined seat surface. The angle of the incline of the head seat surface is such that when the bolt is tightened to a predetermined torque within a threaded hole, the entire seat surface is in contact with the upper surface of an article to be fastened, and a shearing stress is applied to the shank of the bolt in a transverse direction. This shearing stress causes reduced clearance between the threads of the bolt and the threads of the threaded hole, thereby firmly locking the bolt in the hole and preventing loosening of the bolt.

TECHNICAL FIELD

The present invention relates to a bolt protected from loosening itselfon one hand, and also to a method of protecting a bolt from looseningitself on the other hand.

BACKGROUND ART

Some proposals have been made to prevent a bolt set in place from beingloosened unintentionally. In one of the well known proposals, a cutoutis formed in the threaded shank of a bolt and along periphery thereof soas to secure in the cutout an appropriate spring or a resilient plasticspiece.

However, such a proposal would raise manufacture cost due to thesecondary machining of the threaded shank and a considerably intricatework to set the spring or resilient piece in place. Further, thesespring and piece would undesirably cause an additional torque to berequired even to initially fasten such a bolt into a female-threadedhole, thereby rendering uneasy the bolt fastening operations. Inaddition, deterioration of the spring or elastic piece would take placealong the course of time, impairing their effect of locking the bolt.

Primary objects of the present invention, that was made in view of theseproblems in the prior proposals, are to provide a surely lockable boltand a method of surely locking a bolt in place wherein the bolt's headis simply improved as to its seat surface without need of any additionalmember.

SUMMARY OF THE INVENTION

In order to achieve the objects, here is employed the followingtechnical means. Namely, a self-lockable bolt ‘A’ provided herein doescomprises a threaded shank 1 continuing to an enlarged head 2 that has aseat surface 3 gently inclined sideways in one direction. The angle α ofthe inclined seat surface 3 is selected herein within such a range thatan upper side of an article ‘C’ to be tightened with the bolt ‘A’ willbe pressed in a normal direction with its head 2 and simultaneously astress is imparted to the bolt in such a transverse direction as tendingto shear the bolt's shank, when a predetermined torque is applied to thebolt so as to fasten same into a female-threaded hole ‘B’.

The method proposed herein for preventing a bolt from loosening itselfdoes characteristically comprise the steps of preparing a self-lockablebolt ‘A’ having a threaded shank 1 continuing to an enlarged head 2 thathas a seat surface 3 gently inclined sideways in one direction and at anangle α, and then fastening the bolt with a predetermined maximum torqueinto a female-threaded hole ‘B’ to such an extent that due to the angleα an upper side of an article ‘C’ to be tightened with the bolt ‘A’ willbe pressed in a normal direction with the head 2 and simultaneously astress will be imparted to the bolt in such a transverse direction astending to shear the shank 1.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a front elevation of a self-lockable bolt provided in anembodiment of the present invention and partly shown in cross section;

FIG. 2 is a cross section of the bolt that is being fastened;

FIG. 3 is an enlarged cross section of the bolt that has been fastened;and

FIG. 4 is a cross section of a self-lockable bolt provided in anotherembodiment of the present invention.

MODE OF CARRYING OUT INVENTION

Some preferable embodiments of the present invention will now bedescribed referring to the drawings. In the drawings, the referencesymbol ‘A’ denotes a metal bolt having a threaded shank 1 whose upperend is formed integral with a larger-diameter head 2. A bottom surface,i.e., seat face or surface 3 is slanted gently towards one lateral sidein the drawings. An angle at which the seat surface 3 is inclined isselected within a range such that an article ‘C’ to be tightened withthe bolt ‘A’ will be pressed in a normal direction with its head 2 andsimultaneously a stress is imparted to the bolt in such a transversedirection as tending to shear the bolt's shank, when applying apredetermined maximum torque to fasten same into a female-threaded hole‘B’.

The angle α of the inclined seat surface may generally be from about 1.0degree to about 3.0 degrees, though variable depending on the size ofsaid bolt. The predetermined maximum torque referred to above may bethat which corresponds to a standard fastening torque for the usualmetal headed-bolt of the corresponding size, or which may be greaterthan said standard torque by about 10% thereof For examples, thepredetermined torque may be 50-60 Kgf·cm for hexagonally-headed metalbolts ‘M6’, and 400-500 Kgf-cm for similar bolts ‘M12’.

In use of the described bolt ‘A’, it will be fitted in a threaded hole‘B’ and be then twisted initially to such an extent that the slantedseat surface 3 abuts against an upper side of an article ‘C’ that is tobe fastened. Subsequently, the bolt ‘A’ will be further tightened withthe predetermined maximum torque as discussed above. As a result thebolt's head 2 will be forced into a slightly oblique position so thatits slanted seat 3 is finally and entirely set in a surface contact withthe article's upper side as shown in FIG. 3. In this state, an upperregion and a lower region of the threaded shank are pressed on differentinner peripheral zones of the female-threaded hole ‘B’, in oppositetransverse directions as shown at the arrows in FIG. 3. This means thata shearing-stress remains applied to the shank in such a direction astending to shear it transversely, thanks to the forced entire surfacecontact of the slanted seat 3 with the upper side of said article ‘C’.Such an internal stress and strain will reduce clearance between threadsof said bolt ‘A’ and hole ‘B’, thus firmly locking the former not totend to unintentionally loosen within the latter.

The present inventor has prepared samples of ‘M12’ metal bolt having ashank 55 mm long and having a seat surface inclined at an angle of 2.3degrees. A public laboratory has conducted a vibration test on the foursamples and found that they withstood 5 minutes or longer period ofvibration, in contrast with four standard or normal bolts havingloosened within 40 seconds.

Structural details of the embodiment discussed above does never delimitthe scope of the present invention. For example, the smooth inclinedseat surface 3 may be replaced with a series of consecutive steps orwith a continuous series of corrugations. Alternatively, a bolt A′ mayhave an integral flange 4 disposed below its head, with this flangehaving a lower surface 3′ inclined in the same fashion as describedherein above. Any other modifications may be possible provided that thefeatures of the present invention is ensured to achieve the objectsmentioned above and the following advantages are afforded.

In summary the bolt provided herein has a head that is simply improvedto have a seat surface slightly slanted in one direction so that anyconventional bolt former can be used to make such a seat surface at thesame time as forming a body of that bolt. Therefore, the bolt of thepresent invention can now be manufactured at a scarcely raised cost. Theinternal stress and strain produced by fastening the bolt will give riseto a shank shearing force on one hand as well as a normal pressure ontoan article being secured to another article. Consequently, clearancebetween threads of the bolt and the article is reduced to effectivelyprotect the bolt from loosening itself. The easy initial putting of thebolt into the threaded hole will ensure smooth construction or the likeworks, as in the case of ordinary bolts.

What is claimed is:
 1. A locking bolt comprising: a threaded shank,threadingly engageable with a female-threaded hole, the shank having alongitudinal axis and a transverse axis orthogonal to the longitudinalaxis; and a head continuous with the shank and having a seat surfaceengageable with an upper surface of an article to be fastened; whereinthe head seat surface is inclined at an angle of essentially 2.3 degreesfrom the transverse axis of the shank, such that when the shank isthreadingly engaged with the female-threaded hole and a predeterminedtorque is applied to the bolt, the head seat surface is flush with theupper surface of the article and a shearing stress is simultaneouslyapplied to the shank substantially along the transverse axis.
 2. Amethod of locking a bolt in position, the method comprising: providing abolt comprising a threaded shank having a longitudinal axis and atransverse axis orthogonal to the longitudinal axis, and a headcontinuous with the shank having a seat surface inclined at an angle ofessentially 2.3 degrees from the transverse axis of the shank;threadingly engaging the shank with a female-threaded hole; and applyinga predetermined torque to the bolt such that the head seat surface isflush with an upper surface of an article to be fastened and a shearingstress is simultaneously applied to the shank substantially along thetransverse axis.